It seems to me although FB flying interdiction missions get quite good results, their morale collapses very quickly
I fly 3 missions a week, at a height of 21k and rest units whose morale drops below 50.I often find that in the next week only half(or even less) of the units have a morale of 50+
Do you rest the units after the Allied air phase and during both the Allied ground phase and the German turn? That helps.
Unless you're flying into heavy flak, getting heavy interception, or your air bases don't have good supply and support, flying three days a week should leave most of your air groups ready for the next turn.
They are probably diving in from 21k for their actual attacks (they aren't firing rockets and cannon from that height, I assure you), and then getting smacked by flak. The number one eater of allied frontline interdiction is flak, and there's just no getting around taking the hit in some places. Try looking at individual interdiction runs and see whats happening to them.